Hydrocracking is a process which has achieved widespread use in petroleum refining for converting various petroleum fractions to lighter and more valuable products, especially gasoline and distillates such as jet fuels, diesel oils and heating oils.
Hydrocracking is generally carried out in conjunction with an initial hydrotreating step in which the heteroatom-containing impurities in the feed are hydrogenated in the presence of a catalyst with acidic and hydrogenation-dehydrogenation functionality without a substantial bulk conversion of the feed. During this step, the heteroatoms, principally nitrogen and sulfur, are converted to ammonia and hydrogen sulfide; these gases may be removed prior to the subsequent hydrocracking step. However, the two stages may be combined in cascade without interstage separation, for example, as in the Unicracking-IHC Process and as described in U.S. Pat. No. 4,435,275. The presence of large quantities of ammonia in the hydrotreating effluent may result in an undesired suppression of cracking activity although this may be compensated by an increase in severity.
In the hydrocracking step, the hydrotreater effluent is contacted with a hydrocracking catalyst which has both an acidic function and a hydrogenation function. In the first step of the reaction, the polycyclic aromatics in the feedstock are hydrogenated, after which cracking takes place together with further hydrogenation. Depending upon the severity of the reaction conditions, the polycyclic aromatics in the feedstock will be hydrocracked down to paraffinic materials or, under less severe conditions, to monocyclic aromatics as well as paraffins and naphthenes.
The acidic function in the catalysts is provided by a carrier such as alumina, silica-alumina, silica-magnesia or a crystalline zeolite such as faujasite, zeolite X, zeolite Y or mordenite. Large pore zeolites have proved to be highly useful catalysts for this purpose because they possess a high degree of intrinsic cracking activity and, for this reason, are capable of producing a good yield of gasoline. They also possess a better resistance to nitrogen and sulfur compounds than the amorphous materials such as alumina and silica-alumina.
The hydrogenation function is provided by a metal or combination of metals. Noble metals of Group VIIIA of the Periodic Table (the Periodic Table used in this specification is the table approved by IUPAC and the U.S. National Bureau of Standards), especially platinum or palladium may be used, as may base metals of Groups IVA, VIA and VIIIA, especially chromium, molybdenum, tungsten, cobalt and nickel. Combinations of metals such as nickel-molybdenum, cobalt-molybdenum, cobalt-nickel, nickel-tungsten, cobalt-nickel-molybdenum and nickel-tungsten-titanium have been shown to be very effective and useful.
A notable advance in hydrocracking is described in co-pending application Ser. No. 379,421 (and its counterpart, EU 94,827). It was found that one particular zeolite, zeolite beta, had a number of highly useful and characteristic properties when used as the basis for a hydrocracking catalyst. First, it shows a significant distillate selectivity; that is, it tends to produce hydrocracked products boiling in the distillate range (about 165.degree.-345.degree. C., about 330.degree.-650.degree. F.) as opposed to conventional hydrocracking catalysts which are naphtha-directing and which tend to produce a gasoline boiling range (about C.sub.5 to 165.degree. C., about C.sub.5 to 330.degree. F.) product. Although this behavior is shared by other highly siliceous zeolites such as high-silica Y, high silica X and high silica ZSM-20 (as described in EU 98,040), zeolite beta also has the unique ability to hydroisomerize and hydrocrack the paraffinic components of the feed. This is in marked contrast to the behavior of other zeolites such as zeolite Y: if a waxy feedstock is hydrocracked with a conventional large pore catalyst such as zeolite Y, the viscosity of the oil is reduced by cracking most of the 343.degree. C.+ (650.degree. F.+) material into lower boiling products. The remainder of the 345.degree. C.+ material that is not converted, however, contains the majority of the paraffinic components in the feedstock because with these conventional catalysts the aromatics are converted preferentially to the paraffins. The unconverted 345.degree. C.+ material therefore retains a high pour point so that the final, hydrocracked product containing the unconverted paraffins will also have a relatively high pour point. Thus, although the viscosity is reduced, the pour point might still be unacceptable. Even if the conditions are adjusted to give complete or nearly complete conversion, the higher molecular weight hydrocarbons which are present in the feedstock, principally polycyclic aromatics, will be converted to cracking products which include a substantial proportion of straight or slightly branched chain components. If these are of sufficiently high molecular weight themselves (as they often are) they will constitute a waxy component in the product. The final product may therefore be proportionately more waxy than the feedstock (because the non-paraffinic components have been selectively removed by cracking) and, consequently, may have a pour point which is equally unsatisfactory or even more so. Attempts to reduce the molecular weight of these waxy paraffinic products will only serve to produce very light fractions, e.g. propane, so decreasing the desired liquid yield.
Zeolite beta, by contrast, removes the waxy paraffinic components by isomerization and cracking so that a dewaxing effect is achieved simultaneously with the bulk conversion. Consequently, if a gas oil containing paraffins, naphthenes and aromatics is treated under hydrocracking conditions with a zeolite beta catalyst, all three types of hydrocarbon will be converted; other zeolites would selectively hydrocrack the naphthenes and aromatics only, concentrating the paraffins.
The behavior of zeolite beta hydrocracking catalysts is notable not only in that a low pour point distillate product is obtained, but also in that waxy paraffins are removed from the high boiling (345.degree. C.+, about 650.degree. F.+) fraction of the feedstock. This means that the pour point of the high boiling-fraction is also reduced so that it is possible to include a portion of this fraction in the distillate products without violating the specifications--especially the pour point limitations--for the distillate products. In this way, the distillate yield of the process is increased. Generally, it has been found that if the 345.degree.-415.degree. C. (about 650.degree.-775.degree. F.) fraction of the feedstock is dewaxed sufficiently, all of this fraction may be included in the distillate product. Thus, if the dewaxing of this fraction can be maintained at an adequately high level, a substantial improvement in the yield of low pour point distillate may be obtained.